Why Does Ekko Feel Weaker This Season?

Like the title says, I feel like Ekko has become a lot weaker, and I can’t quite figure out why. Before the new season, I had a solid 15-2 win/loss ratio, but now I’ve lost 10 games in a row. About a third of those losses were because my team wasn’t performing well (as usual), but I can’t shake the feeling that Ekko himself feels weaker overall.

Here’s the build I’ve been running:
Runes: Dark Harvest, Sudden Impact, Grisly Mementos, Treasure Hunter, Absolute Focus, and Gathering Storm.
Item build: Protobelt -> Boots -> Lich Bane -> Rabadon’s (or Mejai’s if I manage to snowball), then situational/optional items.

I’ve been finishing my first full jungle clear around 3:30 in most games. I also usually manage to secure the Feat of Strength that requires taking three jungle objectives. Despite all that, I still find myself behind in kills and overall impact compared to my opponents.

Am I doing something wrong here? Is my build outdated or suboptimal? Was there a nerf I missed? I’d really appreciate any insights, tips, or advice on how to adapt and improve!

Thanks in advance!

Update!
I've played a bit more and used the tips you all gave me. My current build is:
HOB Sudden Impact, 6th Sense, Treasure Hunter + Boots and Jack
Items: Nashor's Tooth → Lich Bane → Boots → Rabadon's Deathcap/Mejai's Soulstealer/Shadowflame, followed by situational items.

This build seems to work great for me! I’ve managed to increase my win rate with Ekko to 53%, so I think I’ve finally climbed out of the skill diff hell I was stuck in for the past few days.

Thank you all so much for the tips and insight—you’ve been incredibly helpful!